1. Field of Invention
The present invention relates to an internal combustion engine having a combustion chamber into which inert gas is introduced so as to cause combustion.
2. Description of Related Art
Conventionally, in an internal combustion engine, for example, in a diesel engine, in order to restrict the generation of NOx, an engine exhaust passage and an engine intake passage are connected by exhaust gas recirculation (hereinafter, refer to EGR) so as to recirculate exhaust gas, that is, EGR gas into the engine intake passage via the EGR passage. In this case, since the EGR gas has a relatively high specific heat and accordingly can absorb a large amount of heat, the combustion temperature within the combustion chamber is lowered as an amount of the EGR gas is increased, that is, a rate of the EGR (EGR gas amount/(EGR gas amount+intake air amount)) is increased. When the combustion temperature is lowered, the generation amount of NOx is lowered, so that the greater the EGR rate is increased, the less the generation amount of NOx becomes.
As mentioned above, it has been conventionally known that the generation amount of NOx can be lowered when the EGR rate is increased. However, in the case where the EGR rate is increased, the generation amount of soot, that is, smoke suddenly starts increasing when the EGR rate is over a certain limit. With respect to this point, it has been conventionally considered that the smoke is unlimitedly increased when the EGR rate is increased further, so that it has been considered that the EGR rate at which the smoke suddenly starts increasing is the maximum allowable value of the EGR rate.
Accordingly, the EGR rate has been conventionally defined to stay within a range which is not over the maximum allowable limit. The maximum allowable limit of the EGR rate is significantly different in correspondence to a type of the engine and a fuel, however, is within a range between about 30% and 50%. Therefore, in the conventional diesel engine, the EGR rate is restricted to a range in which about 30% and 50% is the maximum.
As mentioned above, since it has been conventionally considered that the maximum allowable limit exists with respect to the EGR rate, the EGR rate has been defined within the range which does not exceed the maximum allowable limit and so that the generation amount of smoke becomes as little as possible. However, even when the EGR rate is defined so that the generation amount of NOx and smoke becomes as small as possible, a reduction of the generation amount of NOx and smoke has a limit. In fact, the generation of a considerable amount of NOx is inevitable.